I add a calculation just to show the effect of different x values.
This is something to think of for all volumetric machines.
How is this fact handled?
Hi Bo,
Thank you for the interesting question. I have just taken a quick look at the document attached. I am currently away from the office, so can't look at this in detail, but my first comment would be that in the case of waste-heat recovery, where two-phase expansion is an interesting option, assuming a fixed mass-flow rate is not the best assumption. For a particular amount of heat going into the system, reducing the vapour quality increases the mass-flow rate going through the system. Therefore, whilst the specific work may reduce, the total work actually increases.
Interested to hear your thoughts.
Regards,
Martin
Thanks for your comment.
I look at this as a regulation problem.
My question is , how do you handle this in practical life.
Do you accept the generating power or the tip speed to be up and down- depending on the inlet quality ?
I still think that the fluid part passes without generating any work , it is a kind of bypass through the machine. In my case I run with constant speed.
I appreciate the way of handle this fact.
I also show you an email between me and Professor I.K. Smith at City University in London.
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Dear Bo, Many thanks for your email and attachment and my apologies for not replying sooner. Your graphs confirm the fact that the screw is a volumetric machine. It may safely be assumed that the liquid passing through it does a negligible amount of work. Hence, as you increase the dryness fraction but keep the mass flow constant, the volume flow will increase because there is more vapour passing and it is only this which does the work. To pass this through the machine, its speed must increase and hence the power will increase. The only significant effect would be in the pressure drop associated with the filling process. This will decrease as the dryness fraction increases, because the density of the working fluid then decreases and hence the effect of acceleration at the inlet port, which is equal to ρV2 , where ρ corresponds to density and V to fluid velocity, will become less significant. However, by keeping the mass flow constant, this will be countered by the fact that as ρ decreases, V will increase. Kindest regards, Ian
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Looking forward to hear your comments and explanations
Kindest regards,
Bo
I also enclose a file showing that even if the generated effect is the same as a function of inlet quality we can see that the mass flow increases when the quality decreases.
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